Create an API for hosting the Rust runtime

[brson]

It would be nice to have a C API for starting the rust runtime and dynamically injecting schedulers, tasks, etc.

Should really try to get rid of global variables to allow multiple runtimes to be started simultaneously.

Depends on #458, maybe #1682.

[brson]

Interest was expressed in this for purposes of presenting an API for embedding servo.

[brson]

This came up again. Specifically we need to embed Rust in Java.

[pcwalton]

I don't believe this is backwards incompatible, renominating.

[graydon]

just a bug, removing milestone/nomination.

[pnkfelix]

visited for bug triage 17 jul 2013.

[pnkfelix]

visited for bug triage, email from 2013 sep 09. no change otherwise.

[alexcrichton]

We are getting very close to this, but we are not quite there just yet. A working "hello world" case can be found here

[thestinger]

Closing in favour of #11921.